Domestic and commercial refrigeration systems generally use a variety of fluorocarbons and hydrofluorocarbons as refrigerant. Many of the these refrigerants are believed to be responsible for the diminution of the ozone layer above the Earth and legislation is being proposed in many countries to ban or strictly limit the use of such refrigerants.
It has been known for many years that air can be used as a refrigerant. However, refrigeration systems using air have been extremely inefficient compared with refrigeration systems using other refrigerants.
In one historic refrigeration system air was compressed, cooled to room temperature and then expanded to ambient pressure. Typically, the air was compressed to about 100 bar g and, after being cooled to room temperature and expanded through a Joule-Thompson valve to ambient pressure left the Joule-Thompson valve at about -40.degree. C. When applied to commercial refrigeration units, for example the holds of ships carrying food to the colonies, the refrigeration delivered was typically about 0.2 kw refrigeration per kw of energy input. Current systems have been designed using turbo expanders in place of Joule-Thompson valves to reduce the energy consumption. These generally operate with the turbine discharging at close to atmospheric pressure. The refrigeration delivered is typically 0.4 kw refrigeration per kw of energy input. This compares with about 1.25 kw refrigeration per kw of energy input for a modern refrigeration system using a fluorocarbon as refrigerant.
The aim of the present invention is to provide a refrigeration system using air, nitrogen or nitrogen enriched air as the refrigerant and having a power consumption which approaches the power consumption of the modern refrigeration system mentioned above.